Membrane separation, which uses a selective membrane, is a fairly recent addition to the industrial separation technology for processing of liquid streams, such as water purification. In membrane separation, constituents of the influent typically pass through the membrane as a result of a driving force(s) in one effluent stream, thus leaving behind some portion of the original constituents in a second stream. Membrane separations commonly used for water purification or other liquid processing include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), electrodialysis, electrodeionization, pervaporation, membrane extraction, membrane distillation, membrane stripping, membrane aeration, and other processes. The driving force of the separation depends on the type of the membrane separation. Pressure-driven membrane filtration, also known as membrane filtration, includes microfiltration, ultrafiltration, nanofiltration and reverse osmosis, and uses pressure as a driving force, whereas the electrical driving force is used in electrodialysis and electrodeionization. Historically, membrane separation processes or systems were not considered cost effective for water treatment due to the adverse impacts that membrane scaling, membrane fouling, membrane degradation and the like had on the efficiency of removing solutes from aqueous water streams. However, advancements in technology have now made membrane separation a more commercially viable technology for treating aqueous feed streams suitable for use in industrial processes.
During membrane separation, deposits of scale and foulants on the membrane can adversely impact the performance of the membrane. For example, in membrane filtration such foulants and scales can decrease the permeate flow for a given driving force, lowering the permeate quality (purity), increasing energy consumed to maintain a given permeate flow or the like. This can necessitate the cleaning of the membrane separation system in order to remove the scalants, foulants and the like from the membrane separation system. Thus, the performance of the membrane system in use can be enhanced.
In general, the membrane cleaning process includes adding a suitable cleaning agent and circulating it within the membrane separation system. In this regard, the cleaning agent acts to remove sealants, foulants or the like that have deposited on surfaces of the membrane system, including the membrane itself. After the membrane system has been washed with the cleaning agent, the system is then, in general, flushed or rinsed to remove the cleaning agent along with other impurities that may remain in the system.
Membrane cleaning processes usually consist of removing the membrane system from service, rinsing the membrane system (membranes, housings and associated piping) with high quality (preferably permeate quality) water, preparing a cleaning solution by adding the cleaner to a specified volume of permeate quality water, heating the cleaning solution, circulating the cleaning solution at low pressure through the membranes and back into the clean-in-place (CIP) tank thereby displacing the rinse water and diluting the cleaning solutions. The cleaning process further consists of alternately circulating the cleaning solution through the membrane system and soaking the membrane system in the cleaning solution. During the process the system may be rinsed and fresh cleaning solution applied as needed. Finally the system is rinsed with permeate quality water and either subjected to a second cleaning or placed back in service.
Typically, the membrane cleaning process is maintained by evaluating a variety of different process conditions, particularly the pH of the system during cleaning. However, this type of monitoring is not very specific and/or selective to, for example, the concentration of the cleaning agent during cleaning. In this regard, fluctuations in the amount of cleaning agent may not be effectively identified. Thus, the amount of cleaning agent may not be effectively monitored and thereby controlled in order to enhance the performance of the cleaning process.
Accordingly, a need exists to monitor and/or control the cleaning of membrane separation systems where conventional monitoring techniques lack the sensitivity, selectivity and/or accuracy necessary to adequately monitor one or more process parameters specific to the cleaning of membranes or systems in order to adequately evaluate the performance of the same.